1. Field of the Invention
The present invention relates to a porous membrane having a continuous pore structure with a narrow range of pore diameters and no overlapping pores and methods of preparing the porous membrane.
2. Description of Art
Porous membranes have been developed for filtration, ultrafiltration, and other purposes. Almost all the methods capitalize on the phase-inversion mechanism by which a polymer solution is cast (or spun) and thereafter a controlled coagulation process is applied to yield a web-like structure or porous-skin resting on an open cell foam-like structure. Surface pores of such membranes usually exhibit a wide size distribution which can range from 5 nanometers (nm) to several micrometers (.mu.m). Attempts have been made to produce membranes having a continuous porous structure with a well defined, narrow range of pore diameters and with no overlapping pores. One such attempt produced ionotropic gel membranes. Another attempt produced Nucleopore membranes which were ion beam etched to produce pores having well defined dimension and structure. However, overlap of two or more pores always occurred in the latter methods.
In Chem. Abstr. 100(6), 122365b, there is described a porous film membrane made by irradiating a high molecular weight polycarbonate, polyethylene terephthalate, or cellulose nitrate membrane with accelerated charged particles such as accelerated nitrogen ions, etching the membrane with NaOH or NHO.sub.3, and thereby forming pores of homogeneous pore diameter in the membrane. The size of the pores is controlled by changing the kind of ions being accelerated. A polycarbonate membrane is described as having a thickness of 10 microns, a homogeneous pore diameter of 1 micron and a pore density of 2 million/cm.sup.2.
In Chem. Abstr. 101(10), 73901h, there is described a polysulfone porous membrane formed by dissolving more than 5% of a polysulfone resin in trichloroethylene or dimethylformamide, adding methanol or n-hexane to the solution, molding the solution onto a film, and coagulating the molding in a coagulation bath which includes water and/or water-miscible organic liquids, e.g., ethanol. The porous membrane has homogeneous fine pores on the surface having an average pore diameter of 100.degree. A to 5 microns and voids of 30-90%.
In Chem. Abstr. 81 (24). 153976, there is described an acrylonitrile copolymer membrane having uniform fine pores. The membrane is made by extruding an acrylonitrile copolymer solution through a ring die and coagulating with aqueous solutions of different coagulation numbers from inside and outside the tubular film in such a way that at least one of the coagulation solutions has a coagulation number greater than 15.
In Chem. Abstr. 86 (10), 56460t, there is described a porous polymer film of uniform pore size which is produced by pouring a Polymer solution containing a polymer emulsion of 0.05 to 5 microns uniform particle size thereon, wherein the polymer emulsion is produced by seed-polymerization of styrene, vinyltoluene, alphamethylstyrene, or vinyl chloride; contacting the film with a solvent capable of dissolving the polymer emulsion, but not the film, to produce a porous polymer film, the solvent being toluene, propanol, benzene or ethylalcohol.
In Chem. Abstr. 90(20), 153176v, there are described fine porous aromatic polyamide membranes prepared by extruding aromatic polyamides such as polyparaphehylene terephthalamide and polyparabenzamide which have been dissolved in solvents (e g., H.sub.2 SO.sub.4) in the form of film, and orienting the films monoaxially, introducing the films into a coagulation bath containing more than 50% of polymer solvents held below 0.degree. C., and biaxially orienting the films.
In Chem. Abstr., 90(26), 205417w, there is described an ultrafiltration membrane produced from the condensation product of dichloro or diphenyl sulfone and bisphenol, using a non-volatile solvent able to dissolve the condensation product, a C.sub.2 -C.sub.4 polyalcohol and/or water as a solvent. The non-volatile solvent is one or more of m-cresol, chlorobenzene, N,N-dimethyl acetamide and N,N-dimethyl formamide, and the polyalcohol is one or more of ethylene glycol, propylene glycol glycerin and butane diol. The membrane has an unsymmetrical structure with a surface layer having uniform pores and high water permeability.